The TOF-MS includes an interface, a Time-Of-Flight (TOF) region, a gain adjuster, a pulser, a data acquisition circuit, and others. In the TOF-MS, the ionized sample is accelerated and allowed to fly, and a time of flight depending on its mass and an ion intensity (voltage value) are measured, thereby analyzing components contained in the sample.
In an analysis in this TOF-MS, firstly, a sample to be analyzed is ionized in the interface, and is then fed into the TOF region simultaneously with the start of measurement. The ions fed into the TOF region are applied with a voltage at a timing of an ion injection signal, and fly in a predetermined orbit inside the TOF region in a vacuum state.
When ions reach (collide with) a detector in the TOF region, an ion detection signal is outputted from the detector. This ion detection signal is acquired, via the gain adjuster with fixed gain settings, by a data acquisition circuit using an A/D converter, and data of the ion detection signal is outputted to an input/output device via a CPU. The measurement results are displayed as mass spectra, and the components contained in the sample can be analyzed from the intensity (voltage value) of each spectrum and its time (mass).
For example, regarding the TOF-MS, a technology as described in Japanese Patent No. 3701136 (Patent Document 1) has been known, in which gain switching means is provided and the remeasurement for a mass spectrum where an overrange is detected is performed by decreasing the gain, thereby compensating a peak value at which the overrange occurs.
Meanwhile, as a result of studies by the inventors of the present invention for the above-described TOF-MS, the following has been revealed, which is described with reference to FIG. 9 and FIG. 10. FIG. 9 is a drawing that depicts a state of measurement (TOF scan) and an addition process in a mass spectrometer, and FIG. 10 is a drawing of maximum amplitude characteristics of each TOF scan.
For example, in the TOF-MS, the measurement sensitivity (S/N ratio) of spectrum data obtained in one measurement is often insufficient. Therefore, the measurement sensitivity is increased by obtaining a mass spectrum by adding waveform data obtained in plural times of measurement as shown in FIG. 9. In this case, a measurement for obtaining a mass spectrum is referred to as a mass spectrum measurement, and each of the measurements is referred to as a TOF scan. It is assumed herein that the TOF scan is to acquire detector output data of ions accelerated by one ion injection signal, that is, to acquire spectrum data from time t0 (ion injection timing) to time t1 as shown in FIG. 9.
Note that, in FIG. 9, for convenience of description, spectrum data have a peak value at approximately the same time (mass) in any TOF scan. In practice, however, one ion or a plurality of ions are detected at approximately the same time, and therefore the shape of the spectrum (intensity of the peak value and width of the spectrum) is varied in each TOF scan.
A significant feature of the TOF-MS is that a voltage amplitude value of the ion detection signal has a characteristic of being gradually changed based on the number of times of TOF scan (lapse of time). In this case, “gradually” means that the voltage amplitude value is changed by less than twice an amplitude value obtained through the immediately preceding TOF scan, that is, the voltage amplitude value is not abruptly changed.
Changes in a maximum amplitude value of the ion detection signal with respect to the number of times of TOF scan are as shown in FIG. 10. The maximum amplitude value indicates a voltage difference between a maximum peak value and a minimum peak value in spectrum data obtained in one TOF scan (refer to FIG. 9). This maximum amplitude value is proportional to the amount of ions colliding with the detector. That is, the characteristic of FIG. 10 also represents changes in ion concentration during measurement. This characteristic is increased after the start of mass spectrum measurement (ion inflow), and after reaching its peak, it is gradually attenuated. The difference in amplitude between a maximum value and a minimum value is twenty-fold or more in some cases. This is because, although the ion concentration is high immediately after the start of mass spectrum measurement and the number of ions injected through a TOF scan is large, the concentration is gradually lowered by repeating the TOF scan. Also, although there is a slight difference in shape of the characteristic depending on the sample, the degree of vacuum inside the TOF region, etc., the maximum amplitude value is changed gradually or monotonously to some degree as shown in FIG. 10.
Therefore, for example, when a measurement is performed with a full scale (voltage input range) of the A/D converter being matched to the maximum value side of the above-mentioned characteristic, a dynamic range (resolution) of the A/D converter with respect to a signal having a low amplitude value becomes insufficient. Consequently, the measurement accuracy is significantly degraded. Meanwhile, when a measurement is performed with the full scale being matched to the minimum value side, an overrange occurs in the A/D converter with respect to a signal having a large amplitude value, and proper data cannot be obtained. As a result, the measurement accuracy is significantly degraded.
In general, such a problem can be solved by using an A/D converter with an extremely high dynamic range (multi-bit). With such an A/D converter with a high sampling frequency (time resolution), however, the number of bits is increased, which accordingly poses a problem of cost increase.
Also, in the technology disclosed in Patent Document 1, after once obtaining a mass spectrum, an overrange is determined, and then, a measurement is performed again. Therefore, a measurement time is increased.
As described above, in the data acquisition circuit in the mass spectrometer, in order to improve measurement accuracy (S/N ratio) of the ion detection signal, data of TOF scans that have been performed are all acquired, and are then subjected to an addition process. Therefore, a first problem of the conventional technology lies in that signal reproducibility of the added data is significantly degraded when measurement data of a low dynamic range or measurement data where an overrange has occurred are acquired.
Also, a second problem of the conventional technology lies in that, in a method of detecting the occurrence of an overrange signal from the A/D converter and repeating the measurement until the measurement sensitivity reaches a predetermined level, the measurement time until a desired mass spectrum is obtained is increased.